In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendaring rollers at the end of the process. Papermaking devices well known in the art are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is essentially a de-watering system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
In the papermaking process, the major factors at the wire that influence the formation and strength of the paper include: (1) the stock jet speed to wire speed (jet/wire) ratio; (2) the angle that the stock jet lands on the wire; and (3) the rate of water drainage from the web. The speed differential between the stock jet and the wire speed determines the average orientation of the pulp fibers throughout the paper web between the cross, machine, and Z (wet stock height) directions. The average orientation of the fibers within the sheet is critical to both paper formation and sheet strength.
Current machine start-up procedures require optimization of the papermaking machine at different jet/wire ratios and to perform laboratory tests to identify the jet/wire ratio that produces the requisite formation and strength characteristics of the paper. The test results may take several hours and require several trial-and-error changes to the jet/wire ratio before acceptable results are obtained.